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  21 <title>Chapter 7. BIND 9 Security Considerations</title>
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  31 <tr><th colspan="3" align="center">Chapter 7. <acronym class="acronym">BIND</acronym> 9 Security Considerations</th></tr>
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  42 <div class="chapter" lang="en">
  43 <div class="titlepage"><div><div><h2 class="title">
  44 <a name="Bv9ARM.ch07"></a>Chapter 7. <acronym class="acronym">BIND</acronym> 9 Security Considerations</h2></div></div></div>
  45 <div class="toc">
  46 <p><b>Table of Contents</b></p>
  47 <dl>
  48 <dt><span class="sect1"><a href="Bv9ARM.ch07.html#Access_Control_Lists">Access Control Lists</a></span></dt>
  49 <dt><span class="sect1"><a href="Bv9ARM.ch07.html#id2606376"><span><strong class="command">Chroot</strong></span> and <span><strong class="command">Setuid</strong></span></a></span></dt>
  50 <dd><dl>
  51 <dt><span class="sect2"><a href="Bv9ARM.ch07.html#id2606457">The <span><strong class="command">chroot</strong></span> Environment</a></span></dt>
  52 <dt><span class="sect2"><a href="Bv9ARM.ch07.html#id2606517">Using the <span><strong class="command">setuid</strong></span> Function</a></span></dt>
  53 </dl></dd>
  54 <dt><span class="sect1"><a href="Bv9ARM.ch07.html#dynamic_update_security">Dynamic Update Security</a></span></dt>
  55 </dl>
  56 </div>
  57 <div class="sect1" lang="en">
  58 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
  59 <a name="Access_Control_Lists"></a>Access Control Lists</h2></div></div></div>
  60 <p>
  61           Access Control Lists (ACLs) are address match lists that
  62           you can set up and nickname for future use in <span><strong class="command">allow-notify</strong></span>,
  63           <span><strong class="command">allow-query</strong></span>, <span><strong class="command">allow-query-on</strong></span>,
  64           <span><strong class="command">allow-recursion</strong></span>, <span><strong class="command">allow-recursion-on</strong></span>,
  65           <span><strong class="command">blackhole</strong></span>, <span><strong class="command">allow-transfer</strong></span>,
  66           etc.
  67         </p>
  68 <p>
  69           Using ACLs allows you to have finer control over who can access
  70           your name server, without cluttering up your config files with huge
  71           lists of IP addresses.
  72         </p>
  73 <p>
  74           It is a <span class="emphasis"><em>good idea</em></span> to use ACLs, and to
  75           control access to your server. Limiting access to your server by
  76           outside parties can help prevent spoofing and denial of service (DoS) attacks against
  77           your server.
  78         </p>
  79 <p>
  80           Here is an example of how to properly apply ACLs:
  81         </p>
  82 <pre class="programlisting">
  83 // Set up an ACL named "bogusnets" that will block
  84 // RFC1918 space and some reserved space, which is
  85 // commonly used in spoofing attacks.
  86 acl bogusnets {
  87         0.0.0.0/8;  192.0.2.0/24; 224.0.0.0/3;
  88         10.0.0.0/8; 172.16.0.0/12; 192.168.0.0/16;
  89 };
  90
  91 // Set up an ACL called our-nets. Replace this with the
  92 // real IP numbers.
  93 acl our-nets { x.x.x.x/24; x.x.x.x/21; };
  94 options {
  95   ...
  96   ...
  97   allow-query { our-nets; };
  98   allow-recursion { our-nets; };
  99   ...
 100   blackhole { bogusnets; };
 101   ...
 102 };
 103
 104 zone "example.com" {
 105   type master;
 106   file "m/example.com";
 107   allow-query { any; };
 108 };
 109 </pre>
 110 <p>
 111           This allows recursive queries of the server from the outside
 112           unless recursion has been previously disabled.
 113         </p>
 114 </div>
 115 <div class="sect1" lang="en">
 116 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
 117 <a name="id2606376"></a><span><strong class="command">Chroot</strong></span> and <span><strong class="command">Setuid</strong></span>
 118 </h2></div></div></div>
 119 <p>
 120           On UNIX servers, it is possible to run <acronym class="acronym">BIND</acronym>
 121           in a <span class="emphasis"><em>chrooted</em></span> environment (using
 122           the <span><strong class="command">chroot()</strong></span> function) by specifying
 123           the "<code class="option">-t</code>" option for <span><strong class="command">named</strong></span>.
 124           This can help improve system security by placing
 125           <acronym class="acronym">BIND</acronym> in a "sandbox", which will limit
 126           the damage done if a server is compromised.
 127         </p>
 128 <p>
 129           Another useful feature in the UNIX version of <acronym class="acronym">BIND</acronym> is the
 130           ability to run the daemon as an unprivileged user ( <code class="option">-u</code> <em class="replaceable"><code>user</code></em> ).
 131           We suggest running as an unprivileged user when using the <span><strong class="command">chroot</strong></span> feature.
 132         </p>
 133 <p>
 134           Here is an example command line to load <acronym class="acronym">BIND</acronym> in a <span><strong class="command">chroot</strong></span> sandbox,
 135           <span><strong class="command">/var/named</strong></span>, and to run <span><strong class="command">named</strong></span> <span><strong class="command">setuid</strong></span> to
 136           user 202:
 137         </p>
 138 <p>
 139           <strong class="userinput"><code>/usr/local/sbin/named -u 202 -t /var/named</code></strong>
 140         </p>
 141 <div class="sect2" lang="en">
 142 <div class="titlepage"><div><div><h3 class="title">
 143 <a name="id2606457"></a>The <span><strong class="command">chroot</strong></span> Environment</h3></div></div></div>
 144 <p>
 145             In order for a <span><strong class="command">chroot</strong></span> environment
 146             to
 147             work properly in a particular directory
 148             (for example, <code class="filename">/var/named</code>),
 149             you will need to set up an environment that includes everything
 150             <acronym class="acronym">BIND</acronym> needs to run.
 151             From <acronym class="acronym">BIND</acronym>'s point of view, <code class="filename">/var/named</code> is
 152             the root of the filesystem.  You will need to adjust the values of
 153             options like
 154             like <span><strong class="command">directory</strong></span> and <span><strong class="command">pid-file</strong></span> to account
 155             for this.
 156           </p>
 157 <p>
 158             Unlike with earlier versions of BIND, you typically will
 159             <span class="emphasis"><em>not</em></span> need to compile <span><strong class="command">named</strong></span>
 160             statically nor install shared libraries under the new root.
 161             However, depending on your operating system, you may need
 162             to set up things like
 163             <code class="filename">/dev/zero</code>,
 164             <code class="filename">/dev/random</code>,
 165             <code class="filename">/dev/log</code>, and
 166             <code class="filename">/etc/localtime</code>.
 167           </p>
 168 </div>
 169 <div class="sect2" lang="en">
 170 <div class="titlepage"><div><div><h3 class="title">
 171 <a name="id2606517"></a>Using the <span><strong class="command">setuid</strong></span> Function</h3></div></div></div>
 172 <p>
 173             Prior to running the <span><strong class="command">named</strong></span> daemon,
 174             use
 175             the <span><strong class="command">touch</strong></span> utility (to change file
 176             access and
 177             modification times) or the <span><strong class="command">chown</strong></span>
 178             utility (to
 179             set the user id and/or group id) on files
 180             to which you want <acronym class="acronym">BIND</acronym>
 181             to write.
 182           </p>
 183 <div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
 184 <h3 class="title">Note</h3>
 185             Note that if the <span><strong class="command">named</strong></span> daemon is running as an
 186             unprivileged user, it will not be able to bind to new restricted
 187             ports if the server is reloaded.
 188           </div>
 189 </div>
 190 </div>
 191 <div class="sect1" lang="en">
 192 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
 193 <a name="dynamic_update_security"></a>Dynamic Update Security</h2></div></div></div>
 194 <p>
 195           Access to the dynamic
 196           update facility should be strictly limited.  In earlier versions of
 197           <acronym class="acronym">BIND</acronym>, the only way to do this was
 198           based on the IP
 199           address of the host requesting the update, by listing an IP address
 200           or
 201           network prefix in the <span><strong class="command">allow-update</strong></span>
 202           zone option.
 203           This method is insecure since the source address of the update UDP
 204           packet
 205           is easily forged.  Also note that if the IP addresses allowed by the
 206           <span><strong class="command">allow-update</strong></span> option include the
 207           address of a slave
 208           server which performs forwarding of dynamic updates, the master can
 209           be
 210           trivially attacked by sending the update to the slave, which will
 211           forward it to the master with its own source IP address causing the
 212           master to approve it without question.
 213         </p>
 214 <p>
 215           For these reasons, we strongly recommend that updates be
 216           cryptographically authenticated by means of transaction signatures
 217           (TSIG).  That is, the <span><strong class="command">allow-update</strong></span>
 218           option should
 219           list only TSIG key names, not IP addresses or network
 220           prefixes. Alternatively, the new <span><strong class="command">update-policy</strong></span>
 221           option can be used.
 222         </p>
 223 <p>
 224           Some sites choose to keep all dynamically-updated DNS data
 225           in a subdomain and delegate that subdomain to a separate zone. This
 226           way, the top-level zone containing critical data such as the IP
 227           addresses
 228           of public web and mail servers need not allow dynamic update at
 229           all.
 230         </p>
 231 </div>
 232 </div>
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